Abstract: This paper studied the effect of nitrogen fertilizer on water use efficiency(WUE) and relevant parameters of two years old potted apple trees(Starkrimson/ Malus hupenensis) under different soil moisture condition.The results showed that under adequate soil moisture,WUE was decreased with increasing application of nitrogen fertilizer.Nitrogen fertilizer application resulted in an increase of stomatal conductane.Consequently,the transpiration rate was increased more than photosynthetic rate did.Under soil drought,the WUE of plants applied with nitrogen fertilizer was apparently higher than that of control.The WUE value was in the order of high N>medium N>low N.The improvement of WUE was due to the increase of mesophyll capacity,which led to the promotion of photosynthesis.